Method and system for determining a lithology of a subterranean formation

ABSTRACT

A method is provided for determining a lithology of a subterranean formation into which a wellbore has been drilled. The method includes receiving a set of measurement logs including one or more measurement logs, each representing a measured characteristic of the wellbore plotted according to depth. The method also includes segmenting the wellbore into regions based on identified change of trend in one or more of the measurement logs of the set, and sub-segmenting at least one region into zones based on detection of appearance or disappearance of a rock type in the cuttings percentage log, The method also includes determining, in each zone, a location, length and rock type of one or more layers based on a total percentage of each rock type in the zone in the cuttings percentage log and at least one of the additional measurement logs.

BACKGROUND

This application claims priority to and the benefit of EP Application No19306331.0, titled “Method and System for determining a Lithology of asubterranean Formation,” filed Oct. 11, 2019, the entire disclosure ofwhich is hereby incorporated herein by reference.

The disclosure relates to a method and system for determining alithology of a subterranean formation.

When drilling a wellbore, it is critical to know as soon as possible asmany information regarding the wellbore and the surrounding formation inorder to make educated decisions during the drilling of the wellbore andto evaluate the potential of the formation.

In this context, mud logging services are generally used at the wellsite in order to gather information on the wellbore and formation. Mudlogging services comprise in particular sensing of drilling parametersat the surface (such as weight on bit, torque on bit, etc.) as well asmeasurements of the material coming out of the well, in particularanalysis of the cuttings and drilling fluid coming out of the wellbore.

Such services may be complemented by downhole measurements obtaineddirectly by the tool in the formation and transmitted at the surface viatelemetry.

All these measurements enable to perform set up an interpreted lithologyrepresenting the lengths; locations and rock type of the layers in theformation, ie the sequence of the layers drilled. Currently, theinterpreted lithology is created by combining multiple measurements madeby the mud logger while drilling, in particular, the percentage of rocktype (lithology) measured in the drilled cuttings that come to surface,and the data from downhole LWD tools that is transmitted to surface,such as gamma-ray count measurement. The creation of the interpretedLithology is today a manual process that is inconsistent betweendifferent mud loggers and is long and tedious.

SUMMARY

The disclosure relates to a first method for determining a lithology ofa subterranean formation into which a wellbore has been drilled. Themethod comprises receiving a set of measurement logs comprising one ormore measurement logs, each representing a measured characteristic ofthe wellbore plotted according to depth. The measured characteristicinclude at least cuttings percentage and one or more additional measuredcharacteristics. The method also includes segmenting the wellbore intoregions based on identified change of trend in one or more of themeasurement logs of the set, and sub-segmenting at least one region intozones based on detection of appearance or disappearance of a rock typein the cuttings percentage log, The method also includes determining, ineach zone, a location, length and rock type of one or more layers basedon a total percentage of each rock type in the zone in the cuttingspercentage log and at least one of the additional measurement logs.

A plurality of exemplary embodiments of such method are disclosed in thespecification and claims.

The disclosure also relates to a second method for determining alithology of a subterranean formation into which a wellbore has beendrilled. The method comprises receiving a set of measurement logscomprising one or more measurement logs, each representing a measuredcharacteristic of the wellbore plotted according to depth. The measuredcharacteristics include at least cuttings percentage and one or moreadditional measured characteristics. The method also includes segmentingthe wellbore into regions based on identified change of trend in atleast one of the measurement log of the set and automatically generatinga lithology log containing a sequence of layers, each identified by thelocation in depth, the length and the rock type in at least one region.

Automatically generating a lithology log may include sub-segmenting atleast one region into zones based on detection of appearance ordisappearance of a rock type in the cuttings percentage log anddetermining, in each zone, a location, length and rock type of one ormore layers based on a total percentage of each rock type in the zone inthe cuttings percentage log and at least one of the additionalmeasurement logs. In such case, the plurality of exemplary embodimentsdescribed in relationship with the first method are also application tothe second method.

The disclosure also relates to a system for determining a lithology of asubterranean formation into which a wellbore has been drilled. Thesystem comprises a processing system having one or more processorsconfigured to receive a set of measurement logs comprising one or moremeasurement logs. Each log represents a measured characteristic of thewellbore plotted according to depth, and the measured characteristicsinclude at least cuttings percentage and one or more additional measuredcharacteristics. The processing system is also configured to segment thewellbore into regions based on identified change of trend in at leastone of the measurement log of the set and generate a lithology logcontaining a sequence of layers, each identified by the location indepth, the length and the rock type in at least one region.

The above-mentioned systems and methods enable to generate a consistentand accurate deliverable that is not influenced by operator bias orbackground and is available in real-time or near real-time at the wellsite for immediate decision making regarding the drilling operations.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of this disclosure may be better understood upon readingthe following detailed description and upon reference to the drawings inwhich:

FIG. 1 is a schematic drawing of a well site installation including asystem according to an embodiment of the disclosure,

FIG. 2 is a flowchart of a method according to an embodiment of thedisclosure;

FIG. 3 shows measurement logs segmented at an operation of the methodaccording to an embodiment of the disclosure;

FIG. 4 is a plot showing a sub-segmentation of a region of the wellborebased on a cuttings percentage log as per the method according to anembodiment of the disclosure;

FIG. 5 is a flowchart of an operation of the method according to anembodiment of the disclosure,

FIG. 6 is a plot showing a measurement log and a representation of anoperation according to an embodiment of the disclosure,

FIG. 7 is a plot showing two measurement logs and a representation of anoperation according to an embodiment of the disclosure,

FIG. 8 shows a representation of an interpreted lithology log obtainedby the method according to the disclosure and a compared to aninterpreted lithology obtained by an operator based on the samemeasurement logs

DETAILED DESCRIPTION

One or more specific embodiments of the present disclosure will bedescribed below. These described embodiments are examples of thepresently disclosed techniques. Additionally, in an effort to provide aconcise description of these embodiments, some features of an actualimplementation may not be described in the specification. It should beappreciated that in the development of any such actual implementation,as in any engineering or design project, numerousimplementation-specific decisions may be made to achieve the developers'specific goals, such as compliance with system-related andbusiness-related constraints, which may vary from one implementation toanother. Moreover, it should be appreciated that such a developmenteffort might be complex and time consuming, but would still be a routineundertaking of design, fabrication, and manufacture for those ofordinary skill having the benefit of this disclosure.

When introducing elements of various embodiments of the presentdisclosure, the articles “a,” “an,” and “the” are intended to mean thatthere are one or more of the elements. The terms “comprising,”“including,” and “having” are intended to be inclusive and mean thatthere may be additional elements other than the listed elements.Additionally, it should be understood that references to “oneembodiment” or “an embodiment” of the present disclosure are notintended to be interpreted as excluding the existence of additionalembodiments that also incorporate the recited features.’ In all thefollowing, the terms of “upstream” and “downstream” are understoodrelatively to the normal direction of circulation of a fluid in aconduit.

FIG. 1 is a schematic drawing of an installation according to anembodiment of the disclosure.

FIG. 1 is a schematic view of at least a portion of an exampleimplementation of a rotary drilling rig system 5. Downhole measurementscan be conducted by instruments disposed in a drill collar 20. Suchmeasurements may be stored in memory apparatus of the downholeinstruments, or may be telemetered to the surface via conventionalmeasuring-while-drilling (MWD) telemetering apparatus and techniques.For that purpose, an MWD tool sub, schematically illustrated as a tool29, may receive signals from instruments of the collar 20, and maytransmit them via a mud path 8 of a drill string 6 for receipt, e.g.,ultimately via a pressure sensor 14 in a stand pipe 15 and/or to othersurface instrumentation 7.

The drilling rig system 5 may include a motor 2 that may turn a kelly 3through the use of a rotary table 4. The drill string 6 may includesections of drill pipe connected end-to-end to the kelly 3 and may beturned thereby. For example, a plurality of drill collars and/or tools20, 26, 28, and 29 may be attached to the drilling string 6. Suchcollars and tools may collectively form a bottom hole assembly (BHA) 50extending from the drill string 6 to a drilling bit 30.

As the drill string 6 and the BHA 50 turn, the drill bit 30 can bore awellbore 9. An annulus 10 is thus defined between the outside of thedrill string 6 (including the BHA 50) and the wellbore 9 through one ormore subterranean geological formations 32.

A pump 11 may pump drilling fluid (drilling “mud”) from a source, e.g.,from a mud pit 13, via a stand pipe 15, a revolving injector head 17,and the mud path 8 of the kelly 3 and the drill string 6 to the drillbit 30. The mud may lubricate the drill bit 30 and may carry wellborecuttings upward to the surface via the annulus 10. If desired, the mudmay be returned, e.g., to the mud pit 13 or to an appropriate mudregeneration site, where it may be separated from cuttings and the like,degassed, and returned for application again to the drill string 6.

Separating the cuttings from the mud is performed via shale shakers 52.Once the mud and the cuttings have been separated, they may be collectedand analyzed. Cuttings samples 54 may be collected manually and analyzedin a mud logging cabin (not represented) with one or more instruments(such as microscope, X-ray fluorescence (XRF), X-Ray Diffraction (XRD),and the like). Alternatively, the cuttings sample may be collected andanalyzed automatically at the well site. Regarding the mud (or drillingfluid), it is generally sampled at the outlet of the shakers by asampling device 56 and directed to an extractor 58 that extracts gasfrom the mud. The gas is then directed to an analyzer 60 (such asThermal Conductivity Detector (TCD), Flame Ionization Detector (FID) ormass spectrometer) in order to detect the content of one or more gas,optionally with the interposition of a gas chromatograph.

The downhole tool (collar) 20 may be any type of downhole tool takingmeasurement, such as an ultrasonic tool, an electromagnetic orresistivity tool, a sampling tool. For example, the ultrasonic tool 20may include at least one or more sensors 45, 46, e.g., such as formeasuring characteristics of the wellbore 9 and/or fluid, includingpressure, standoff, composition, etc. therein during drillingoperations. Such measurements may be conducted while the wellbore 9 isbeing drilled and/or with the drill string 6 and the BHA 50 in thewellbore 9 while the drill bit 30, the BHA 50, and the drill string 6are not rotating. Such measurements may be conducted while the drillstring 6, the BHA 50, and the drill bit 30 are being tripped to and fromthe bottom of the wellbore 9. The measurements (or data based at leastpartially thereon) may be transmitted to the surface via the MWDtelemetry tool 29 and the internal mud passage 8 of the drill string 6(or the annulus 10), or they may be recorded and stored downhole and forretrieval at the surface after the drill string 6 and BHA 50 have beenremoved from the wellbore 9.

The sensors 45, 46 may be mounted on stabilizer fins 27 of the downholetool 20, as depicted in FIG. 1 , or may be mounted in a cylindrical wall23 of the downhole tool 20.

An electronics module 22 may contain electronic circuits,microprocessors, memories, and/or the like, operable to control, and/orto receive, process, and/or store data from the sensors 45, 46, whichmay be mounted on a sleeve, an inner tube, and/or other section 21secured around or within the collar of the ultrasonic tool 20. Thesection 21 and other components of the BHA 50 may include a path 40 bywhich drilling mud may pass through the interior passage 8 of the drillstring 6 to the drill bit 30.

A portion of the drilling rig system 5, such as surface instrumentation7, may include other sensors for measurement parameters at the surface,such as flow, pressure, weight on bit, torque on bit, etc. and verifythat the system works properly. As an example, a sensor 11′ may beconnected to the pump 11 to count the number of strokes of the pump, asensor 3′ may be present at the Kelly or motor to assess the rotationsper minute (RPM) or in the weight and torque on bit.

The surface instrumentation 7 may also include data processing system 8that can encompass one or more, or portions thereof, of the following:control devices and electronics in one or more modules of the BHA 50(such as a downhole controller), a remote computer system (not shown),communication equipment, and other equipment. The data processing systemmay include one or more computer systems or devices and/or may be adistributed computer system. For example, collected data or informationmay be stored, distributed, communicated to a human wellsite operator,and/or processed locally or remotely.

The data processing system may, individually or in combination withother system components, is also linked to all or part of the sensors,downhole or at the surface, to process the measurements and may performthe methods and/or processes described below, or portions thereof. Forexample, such data processing system may include processor capabilityfor collecting data obtained from the sensors at the surface ordownhole. Methods and/or processes within the scope of the presentdisclosure may be implemented by one or more computer programs that runin a processor located, e.g., in one or more modules of the BHA 50and/or surface equipment of the drilling rig system 5. Such programs mayutilize data received from the BHA 50 via mud-pulse telemetry and/orother telemetry means, and/or may transmit control signals to operativeelements of the BHA 50. The programs may be stored on a tangible,non-transitory, computer-usable storage medium associated with the oneor more processors of the BHA 50 and/or surface equipment, such assurface instrumentation 7, of the drilling rig system 5, or may bestored on an external, tangible, non-transitory, computer-usable storagemedium electronically coupled to such processor(s). The storage mediummay be one or more known or future-developed storage media, such as amagnetic disk, an optically readable disk, flash memory, or a readabledevice of another kind, including a remote storage device coupled over acommunication link, among other examples.

A method 100 for determining lithology of the formation is described inreference with FIGS. 2-8 .

The method first comprises receiving (block 102) a set of measurementlogs comprising one or more measurement log. Each of the measurement logrepresents a measured characteristic of the wellbore plotted accordingto depth as can be seen on FIG. 3 . The measurements log of the setcomprise a cuttings percentage, obtained from the sampling of cuttingsextracted from the wellbore at the surface, and one or more additionalmeasurements, taken at the surface or downhole. Such additionalmeasurement log may be directly sensed at the wellsite (for instance,weight on bit, torque on bit, total gas in mud obtained from the gasanalyzer or gamma-ray count obtained from the downhole tool) or computedfrom a combination of sensed parameters (for instance, a formationstrength). The method may therefore also comprise computing one of moreof the measurements that are not directly obtained from sensedparameters. Concerning the formation strength, for instance, it maygenerally be computed from the weight on bit, rate of penetration andrevolution per minutes of the drill bit.

One or more measurement logs of the set are then segmented (block 104)based on change of trend in the log. We can see examples of measuredcharacteristics being total gas in mud 105A, gamma-ray count 105B,cuttings percentage 105C and formation strength 105D. In each log, thereare many variations of the measurement from which the general trends 106are extracted. The points 107 situated at the change or inflexion of atrend is considered as a “change point” and its depth is considered asan extremity of a region. Therefore, the measurement log is segmentedinto regions based on the trend analysis.

The trend analysis and “change point” determination may be obtained byusing a “Change Point’ algorithm. This algorithm allows a detection ofthe edges. It is based on Bayesian approach but the kind of approach isnot limited other approaches for segmenting the log may be used. The“Change Point” algorithm is disclosed in more details in patentapplication WO2010/043951.

The trend analysis may be run on one log, such as rate of penetration,total drilled gas, gamma ray from MWD, and any LWD measurements such asresistivity when available.

For more robustness, the trend analysis may also be performed on aplurality of measurement logs 105A-105D as represented on FIG. 3 . Inthis case, all “change points” may not be selected and some conditionsmay be apply, such as selecting a first “change point” depth in a firstlog as an extremity of a region if at least another log show a “changepoint’ in a predetermined depth interval around the first “change point’depth. In this case, the other “change point” in the depth interval maybe deleted. Alternatively, a measurement log may be associated to aconfidence in view of the type of measurement and of the way themeasurement was performed and the measurement with highest confidencemay be chosen as reference measurement and its ‘change point’ may beused for determining the regions.

The method then comprises generating an interpreted lithology log (block110) in at least one region of the wellbore. Therefore, the interpretedlithology may be obtained in a consistent way, in real-time. Inparticular, the method may be configured to automatically generate theinterpreted lithology log. By “automatically”, it is meant that theoperations performed as part of block 110 are performed without directhuman control. A human may set parameters or validate the results of theautomatic generation but does not need to intervene so that theoperations described herebelow are performed.

The generation comprises sub segmenting (block 112) the region intozones based on rock type appearance and disappearance in the cuttingspercentage log as represented on FIG. 4 , showing a region of thecuttings percentage log 114. The boundary of the zone corresponds to thedepth of the rock type appearance or disappearance. In other words azone ends and a new zone starts when a new type of cuttings appears onthe cuttings percentage log or disappears from the cuttings percentagelog. For instance, as can be seen on FIG. 4 , there is only a first rocktype (here, clay-s) in a upper portion of the log 114. This portionforms a first zone 116 that ends at depth 113 when a second type of rocktype (here limestone) appears in the cuttings percentage log. The secondzone 118 starts at 113 and ends at depth 115 when a third rock type(such as dolomite) appears, starting a third zone 119. The third zone118 ends at 117 when the third rock type disappears from the log.

Then, in each zone, the method includes determining (block 120) thelocation, length and rock type of one or more layers based on total rocktype percentage in the zone and at least one additional measuredcharacteristic (i.e. measurement). This operation is represented in moredetails in FIG. 5 .

First, the operation 120 comprises computing (block 122) the totalpercentage of each rock type in a zone. This may be performed forinstance by computing the area between two curves on the measurementlog, that is representative of the measurement. For instance in zone114, the percentage of the first rock type would be 100% while thepercentage of second and third rock type would be 0%. In zone 116, thetotal percentage of second rock type would be defined by the area 122and the total percentage of first rock type by the area 124, bothdivided by the total area of the zone.

The method then comprises determining (block 124) the aggregated lengthof the layers having a predetermined rock type. In the following the“length” of a layer is defined as the difference between the depths itextends. For instance the length of the zone 118 is the differencebetween depth 113 and depth 115. The “aggregated length” of the layersis the sum of their respective length. Determining the aggregated lengthof the layers includes determining a length corresponding to apercentage of the zone length equal to the total percentage of thepredetermined rock type in the zone, ie multiplying the length of thezone by the total percentage for the rock type. The aggregated length isused to preserve the distribution of cuttings percentage. For example,if a zone has a length of 50 m, and there 20% of clay in the zone, theaggregated length of the clay layers, whether in one or several layers,is 10 m.

The method also comprises determining (block 126) a location of one ormore layers having the predetermined rock type. This includesdetermining a set of depths in the zone for which the values of one ofthe additional measurement are closest to an extremum of the measurementin the zone. The set of depths is determined so that its aggregatedlength matches the aggregated length of the layers. The additionalmeasurement may be any additional measurement that is consideredrelevant. For instance, for clay or shale, the selected additionalmeasured characteristic may be gamma-ray count, for limestone, theselected additional measured characteristic may be formation strengthand for sandstone, the selected additional measured characteristic maybe total gas in mud. For these three additional measuredcharacteristics, the location of the layers is detected based on a setof depths closest to a maximum of the log. The disclosure is however notlimited to this set of additional measured characteristics and any otherrelevant additional measured characteristic may be selected. Further,the minimum of the corresponding log of such characteristic may also beselected for attributing the location of the layers.

FIG. 6 shows this operation in more detail. FIG. 6 shows a measurementlog 130 having an additional measured characteristic 132 as a functionof depth 134 and enables to identify a first predetermined rock type.For instance, as represented on FIG. 6 , this operation includesidentifying a key value 136 for which the depth intervals (ie set ofdepths) of data points having values superior to the key valuecorresponds to the aggregated length of the layers. This may beperformed by flagging local maxima 138-144 and calculating the depthinterval of the data points having values superior to all local maximaexcept for one (see interval 146 on FIG. 6 ). It corresponds to alocation of a first layer. This depth interval may be compared with theaggregated length of the layers of the predetermined rock type computed.If the interval is shorter than the aggregated length that has beencomputed, the location of a second layer will be set around the secondhighest maximum 138 and the sum of the depth intervals having datapoints having values superior to all local maxima except for two may becalculated (intervals 148A, 148B). If the interval is shorter than theaggregated length of the layers of the predetermined rock type that hasbeen computed, the operation is renewed with the third local maxima. Ifthe sum of depth intervals is longer than the aggregated length, the keyvalue 136 may be search between the second and third highest maxima 138and 140, for instance using squeezing techniques. The set of depths isdefined as all depths having values being superior to the key value, inparticular on FIG. 6 , intervals 148A, 148B.

Preferably, as several rock type are present in the formation, once thelocations of the layers of the predetermined rock type, designated firstpredetermined rock type, have been determined, the location of thelayers of an another rock type, ie second rock type, are determined.

Therefore, the method includes defining (block 150) an updated zone sothat the updated zone comprises the length of the initial zone exceptfor the set of depths that has been selected as the location of thefirst rock type. The updated zone is shown on FIG. 6 as 152.

The method is then reset with a second predetermined rock type and theupdated zone, ie the aggregated length of the layers having second rocktype is determined, and a second set of depths corresponding to thelocation of these layers is determined. The second set of depths may bedetermined based on the same additional measurement that has been usedfor the first predetermined rock type, or based on a differentadditional measurement.

As can be seen on FIG. 7 , showing two additional measurements 130, 160plotted on the same chart as a function of depth 162, the firstadditional measurement already represented on FIG. 6 that enabled todetermine the locations of layers of the first predetermined rock typeand a second additional measurement enabling to determine the secondrock type, the same operation of determining the location of the layersby determining the set of depths closest to an extremum is performedagain but in the updated zone 152, discarding the data points that arein the depth intervals 148A, 148B. In this scenario, there is only onelayer of the second predetermined rock type at the depth interval 154.

These operations 124, 126, 150 may be performed iteratively until nomeasurement is available to identify a particular rock type. It can beadaptative depending on the number and type of measurement available inthe well and some parameters relative to the well, for instance itsgeometry or the region in which it is disposed. When no measurement isavailable anymore, an operator may be consulted to complete theinterpreted lithology log or the location of the remaining layers may bedetermined based on the remaining locations.

A sequence that could be used for determining the locations of layersmay be the following:

-   -   The first predetermined rock type is clay and shale and the        locations of the layers having this rock type is determined        based on gamma-ray measurement maxima,    -   The second predetermined rock type is limestone and the        locations of the layers having this rock type is determined        based on formation strength measurement maxima,    -   The third predetermined rock type is sandstone and the locations        of the layers having this rock type is determined based on total        gas in mud measurement maxima.

As previously indicated, this sequence is an exemplary one and othersequences may be used within the scope of the disclosure.

The method then comprises generating (block 170) the interpretedlithology log based on the location, length and rock type of the layers,corresponding to plot the location of the layers as a function of depth.Such a log 180 is represented on FIG. 8

The method as per the disclosure has been tested with theabove-mentioned on a well site. The figure shows cuttings percentage172, Formation strength 174, rate of penetration (ROP) 176, total gas(TG) 177, mechanical specific energy (MSE) 178, gamma-ray (GR) 179, allparameters that may be used in the determination of the interpretationlithology log. The figure shows as well an interpreted lithology log 180obtained by the above-mentioned method and compared with the interpretedlithology as established by the operator on site 182 as well asmineralogy obtained by downhole tool 184. The mineralogy shows (quartz,feldspath, mica) at 185, the carbonate at 189 and clay at 187. Theresults 180 obtained by the method are more coherent and compact thanthe results from the operators 182, as can be seen from comparison withLWD mineralogy. For instance, in zone 186, a dolomite zone is visible inthe log 180 and this zone is perfectly aligned with the carbonate zonein the log 184. Similarly in zone 188, the log 180 shows a zone ofdolomite that is in agreement with what is shown on log 184. In zone190, the LWD log 184 shows a zone of clay that matches with the clayzone that is obtained with the above-mentioned method.

We can see that both interpreted lithology logs are very similar. Themethod according to the disclosure enabling to obtain the interpretedlithology early on, in real-time or near real-time, enables to have moreinformation regarding the formation and to take more educated drillingdecisions as soon as possible.

The disclosure relates to a first method for determining a lithology ofa subterranean formation into which a wellbore has been drilled. Themethod comprises receiving a set of measurement logs comprising one ormore measurement logs, each representing a measured characteristic ofthe wellbore plotted according to depth. The measured characteristicinclude at least cuttings percentage and one or more additional measuredcharacteristics. The method also includes segmenting the wellbore intoregions based on identified change of trend in one or more of themeasurement logs of the set, and sub-segmenting at least one region intozones based on detection of appearance or disappearance of a rock typein the cuttings percentage log, The method also includes determining, ineach zone, a location, length and rock type of one or more layers basedon a total percentage of each rock type in the zone in the cuttingspercentage log and at least one of the additional measurement logs.

A plurality of exemplary embodiments of such method are disclosed in thespecification and claims.

In an embodiment, the additional measured characteristics comprise oneor more of a gamma-ray count, a rate of penetration, a total gas in mud,a formation strength, a weight on bit, or a resistivity. In particular,when the additional measured characteristics comprise the formationstrength, the method comprises computing the formation strength from theweight on bit, revolutions per minute and rate of penetration

In an embodiment, determining the length of one or more layers having apredetermined rock type in the zone includes determining an aggregatedlength of layers having the predetermined rock type, wherein theaggregated length corresponds to percentage of the zone length equal tothe total percentage of the predetermined rock type in the zone. Inparticular, determining a location of one or more layers having thepredetermined rock type include determining a set of depths in the zone,the set of depths being defined so that the corresponding values for atleast one of the additional measured characteristics are closest to anextremum of the additional measured characteristic in the zone, whereinthe set of depths is determined so that its aggregated length matchesthe aggregated length of the layers having the predetermined rock type

In the above-mentioned embodiment, the method may include determiningthe aggregated length of layers for each rock type, determining thelocation of the one or more layers having a first predetermined rocktype in the zone, creating an updated zone consisting of the zoneexcluding the set of depths corresponding to the first predeterminedrock type and determining the location of the one or more layers havinga second predetermined rock type in the updated zone. In particular,determining the location of the one or more layers having the firstpredetermined rock type in the zone may be based on a first additionalmeasured characteristic and determining the location of the one or morelayers having the second predetermined rock type in the updated zone isbased on a second additional measured characteristic.

In a particular embodiment, determining the location of one or morelayers having the predetermined rock type may include selecting the setof depths according to one or more of the following:

-   -   The additional measured characteristics including gamma-ray        count, the set of depths having gamma-ray count values closest        to a maximum is selected when the predetermined rock type is        clay and shale,    -   The additional measured characteristics including a formation        strength, the set of depths having formation strength values        closest to a maximum is selected when the predetermined rock        type is limestone,    -   The additional measured characteristics including a formation        total gas in mud, the set of depths having total gas in mud        values closest to a maximum when the predetermined rock type is        sandstone.

In an embodiment, if a first predetermined rock type is clay and shale,the method may determine a first set of depths in the zone havinggamma-ray count values closest to a maximum. If a second predeterminedrock type is limestone, the method may determine a second set of depthshaving formation strength values closest to a maximum in a first updatedzone consisting of the zone excluding the first set of depths. If athird predetermined rock type is sandstone, the method may determine athird set of depths in a second updated zone consisting of the firstupdated zone excluding the second set of depths.

In an embodiment, segmenting the measurements log includes identifyingchange points in the measurement log, wherein at least a change pointdepth is defined as a boundary of a region. In such case, segmenting themeasurements may include identifying first change points in a firstmeasurement log and second change points in a second measurement log andselecting a depth of a first change point as a boundary of a region if asecond change point is identified in a predetermined depth intervalaround said depth.

In an embodiment, the method includes generating a lithology log basedon the location, length and rock type of the one or more layers

The disclosure also relates to a second method for determining alithology of a subterranean formation into which a wellbore has beendrilled. The method comprises receiving a set of measurement logscomprising one or more measurement logs, each representing a measuredcharacteristic of the wellbore plotted according to depth. The measuredcharacteristics include at least cuttings percentage and one or moreadditional measured characteristics. The method also includes segmentingthe wellbore into regions based on identified change of trend in atleast one of the measurement log of the set and generating a lithologylog containing a sequence of layers, each identified by the location indepth, the length and the rock type in at least one region. Thegeneration of the lithology log is preferably made automatically, iewithout direct human control.

Generating a lithology log may include sub-segmenting at least oneregion into zones based on detection of appearance or disappearance of arock type in the cuttings percentage log and determining, in each zone,a location, length and rock type of one or more layers based on a totalpercentage of each rock type in the zone in the cuttings percentage logand at least one of the additional measurement logs. In such case, theplurality of exemplary embodiments described in relationship with thefirst method are also application to the second method.

The disclosure also relates to a system for determining a lithology of asubterranean formation into which a wellbore has been drilled. Thesystem comprises a processing system having one or more processorsconfigured to receive a set of measurement logs comprising one or moremeasurement logs. Each log represents a measured characteristic of thewellbore plotted according to depth, and the measured characteristicsinclude at least cuttings percentage and one or more additional measuredcharacteristics. The processing system is also configured to segment thewellbore into regions based on identified change of trend in at leastone of the measurement log of the set and generate a lithology logcontaining a sequence of layers, each identified by the location indepth, the length and the rock type in at least one region. Theprocessing system may be configured to generate automatically thelithology log, ie without direct human control.

The system may comprise at least one of a cuttings sample analysisdevice for analyzing cuttings exiting the wellbore and at least one of agas sample analysis device for analyzing gas extracted from the drillingfluid exiting the wellbore, wherein one of the additional measuredcharacteristic is total gas in mud; a downhole tool for taking one ormore downhole measurement, wherein one of the additional measuredcharacteristic is gamma-ray count or resistivity; and a sensor situatedat the well site, at the surface, to measure one or more of theparameters relative to a drilling installation of the wellbore, whereinone of the additional measured characteristic is weight on bit, torqueon bit, rate of penetration, rotation per minute or formation strength.

Further, the processing system may be configured to sub-segment at leastone region into zones based on detection of appearance or disappearanceof a rock type in the cuttings percentage log, and, in each zone,determine a location, length and rock type of one or more layers basedon a total percentage of each rock type in the zone in the cuttingspercentage log and at least one of the additional measurement logs.

The invention claimed is:
 1. A method for determining a lithology of asubterranean formation into which a wellbore has been drilled,comprising: receiving a set of measurement logs comprising a pluralityof measurement logs, each measurement log representing a measuredcharacteristic of the wellbore plotted according to depth, wherein themeasured characteristics of the set of measurement logs include cuttingspercentage and one or more additional measured characteristics,segmenting the wellbore into regions based on an identified change oftrend in at least one of the measurement logs of the set, whereinsegmenting the wellbore includes identifying first change points in afirst measurement log and second change points in a second measurementlog and selecting a depth of a first change point as a boundary of aregion if a second change point is identified in a predetermined depthinterval around the depth of the first charge point, sub-segmenting atleast one region into zones based on a detection of appearance ordisappearance of a rock type in the cuttings percentage log, and in eachzone, determining a location, length and rock type of one or more layersbased on a total percentage of each rock type in the zone in thecuttings percentage log and at least one additional measurement log. 2.The method of claim 1, wherein the additional measured characteristicscomprise one or more of a gamma-ray count, a rate of penetration, atotal gas in mud, a formation strength, a weight on bit, or aresistivity.
 3. The method of claim 2, wherein the additional measuredcharacteristics comprise the formation strength, wherein the methodfurther comprises computing the formation strength from the weight onbit, revolutions per minute and rate of penetration.
 4. The method ofclaim 1, wherein determining the length of the one or more layersincludes determining an aggregated length of layers having apredetermined rock type, wherein the aggregated length corresponds to apercentage of the zone length equal to a total percentage of thepredetermined rock type in the zone.
 5. The method of claim 4, whereindetermining the length of the one or more layers further includesdetermining a location of one or more layers having the predeterminedrock type, wherein determining the location includes determining a setof depths in the zone, the set of depths being defined so that thecorresponding values for at least one of the additional measuredcharacteristics are closest to an extremum of the additional measuredcharacteristic in the zone, wherein the set of depths is determined sothat its aggregated length matches the aggregated length of the layershaving the predetermined rock type.
 6. The method of claim 5, whereindetermining the location of the one or more layers having thepredetermined rock type further includes selecting the set of depthsaccording to one or more of the following: the additional measuredcharacteristics including gamma-ray count, the set of depths havinggamma-ray count values closest to a maximum is selected when thepredetermined rock type is clay and shale, the additional measuredcharacteristics including a formation strength, the set of depths havingformation strength values closest to a maximum is selected when thepredetermined rock type is limestone, and the additional measuredcharacteristics including a formation total gas in mud, the set ofdepths having total gas in mud values closest to a maximum when thepredetermined rock type is sandstone.
 7. The method of claim 4, whereinthe predetermined rock type is a first predetermined rock type, andwherein determining the length of the one or more layers furtherincludes creating an updated zone consisting of the zone excluding theset of depths corresponding to the first predetermined rock type anddetermining the location of the one or more layers having a secondpredetermined rock type in the updated zone.
 8. The method of claim 7,wherein determining the location of the one or more layers having thefirst predetermined rock type in the zone is based on a first additionalmeasured characteristic and determining the location of the one or morelayers having the second predetermined rock type in the updated zone isbased on a second additional measured characteristic.
 9. The method ofclaim 7, wherein the first predetermined rock type is clay and/or shaleand the method further includes determining a first set of depths havinggamma-ray count values closest to a maximum in the zone associated tothe first predetermined rock type; and wherein the second predeterminedrock type is limestone, and the method further includes determining asecond set of depths having formation strength values closest to amaximum in a first updated zone consisting of the zone excluding thefirst set of depths.
 10. The method of claim 9, wherein a thirdpredetermined rock type is sandstone and the method further includesdetermining a third set of depths in a second updated zone consisting ofthe first updated zone excluding the second set of depths.
 11. Themethod of claim 1, further comprising generating a lithology log basedon the location, length and rock type of the one or more layers.
 12. Amethod for determining a lithology of a subterranean formation intowhich a wellbore has been drilled, comprising: receiving a set ofmeasurement logs comprising a plurality of measurement logs, eachmeasurement log representing a measured characteristic of the wellboreplotted according to depth, wherein the measured characteristics of theset of measurement logs include cuttings percentage and one or moreadditional measured characteristics, segmenting the wellbore intoregions based on an identified change of trend in at least one of themeasurement logs of the set, sub-segmenting at least one region intozones based on a detection of appearance or disappearance of a rock typein the cuttings percentage log, and in each zone, determining alocation, length and rock type of one or more layers based on a totalpercentage of each rock type in the zone in the cuttings percentage logand at least one additional measurement log, wherein determining thelength of the one or more layers includes: determining an aggregatedlength of layers having a predetermined rock type, wherein theaggregated length corresponds to a percentage of the zone length equalto a total percentage of the predetermined rock type in the zone; anddetermining a location of one or more layers having the predeterminedrock type, wherein determining the location of the one or more layershaving the predetermined rock type includes determining a set of depthsin the zone, the set of depths being defined so that the correspondingvalues for at least one of the additional measured characteristics areclosest to an extremum of the additional measured characteristic in thezone, wherein the set of depths is determined so that its aggregatedlength matches the aggregated length of the layers having thepredetermined rock type.
 13. The method of claim 12, wherein determiningthe location of the one or more layers having the predetermined rocktype further includes selecting the set of depths according to one ormore of the following: the additional measured characteristics includinggamma-ray count, the set of depths having gamma-ray count values closestto a maximum is selected when the predetermined rock type is clay andshale, the additional measured characteristics including a formationstrength, the set of depths having formation strength values closest toa maximum is selected when the predetermined rock type is limestone, andthe additional measured characteristics including a formation total gasin mud, the set of depths having total gas in mud values closest to amaximum when the predetermined rock type is sandstone.
 14. A method fordetermining a lithology of a subterranean formation into which awellbore has been drilled, comprising: receiving a set of measurementlogs comprising a plurality of measurement logs, each measurement logrepresenting a measured characteristic of the wellbore plotted accordingto depth, wherein the measured characteristics of the set of measurementlogs include cuttings percentage and one or more additional measuredcharacteristics, segmenting the wellbore into regions based on anidentified change of trend in at least one of the measurement logs ofthe set, sub-segmenting at least one region into zones based on adetection of appearance or disappearance of a rock type in the cuttingspercentage log, and in each zone, determining a location, length androck type of one or more layers based on a total percentage of each rocktype in the zone in the cuttings percentage log and at least oneadditional measurement log, wherein determining the length of the one ormore layers includes determining an aggregated length of layers having apredetermined rock type, wherein the aggregated length corresponds to apercentage of the zone length equal to a total percentage of thepredetermined rock type in the zone, and wherein the predetermined rocktype is a first predetermined rock type, and wherein determining thelength of the one or more layers further includes creating an updatedzone consisting of the zone excluding the set of depths corresponding tothe first predetermined rock type and determining the location of theone or more layers having a second predetermined rock type in theupdated zone.
 15. The method of claim 14, wherein determining thelocation of the one or more layers having the first predetermined rocktype in the zone is based on a first additional measured characteristicand determining the location of the one or more layers having the secondpredetermined rock type in the updated zone is based on a secondadditional measured characteristic.
 16. The method of claim 14, whereinthe first predetermined rock type is clay and/or shale and the methodfurther includes determining a first set of depths having gamma-raycount values closest to a maximum in the zone associated to the firstpredetermined rock type; and wherein the second predetermined rock typeis limestone, and the method further includes determining a second setof depths having formation strength values closest to a maximum in afirst updated zone consisting of the zone excluding the first set ofdepths.
 17. The method of claim 16, wherein a third predetermined rocktype is sandstone and the method further includes determining a thirdset of depths in a second updated zone consisting of the first updatedzone excluding the second set of depths.
 18. The method of claim 14,wherein the additional measured characteristics comprise one or more ofa gamma-ray count, a rate of penetration, a total gas in mud, aformation strength, a weight on bit, or a resistivity.
 19. The method ofclaim 14, wherein the additional measured characteristics comprise theformation strength, wherein the method further comprises computing theformation strength from the weight on bit, revolutions per minute andrate of penetration.
 20. The method of claim 14, further comprisinggenerating a lithology log based on the location, length and rock typeof the one or more layers.